Y.-K. Ko

An empirical study of the electron temperature and heavy ion velocities in the south polar coronal hole

The solar wind ions flowing outward through the solar corona generally have their ionic fractions ‘freeze-in’ within 5 solar radii. The altitude where the freeze-in occurs depends on the competition between two time scales: the time over which the wind flows through a density scale height, and the time over which the ions achieve ionization equilibrium. Therefore, electron temperature, electron density, and the velocity of the ions are the three main physical quantities which determine the freeze-in process, and thus the solar wind ionic charge states. These physical quantities are determined by the heating and acceleration of the solar wind, as well as the geometry of the expansion. In this work, we present a parametric study of the electron temperature profile and velocities of the heavy ions in the inner solar corona. We use the ionic charge composition data observed by the SWICS experiment on Ulysses during the south polar pass to derive empirically the electron temperature profile in the south polar coronal hole. We find that the electron temperature profile in the solar inner corona is well constrained by the solar wind charge composition data. The data also indicate that the electron temperature profile must have a maximum within 2 solar radii. We also find that the velocities of heavy ions in their freeze-in regions are small (<100 km s-1) and different elements must flow at different velocities in the inner corona.

Limitations on suprathermal tails of electrons in the lower solar corona

Measurements of the charge states of solar wind ions by the SWICS instrument on Ulysses are used to place limits on the extent to which the velocity distribution of electrons in the lower solar corona deviates from a Maxwellian distribution by having a suprathermal tail. It is found that the data are consistent with a suprathermal tail which is not large, and which has a minor influence on the location and magnitude of the maximum of the electron temperature. This result has implications for theories which propose that suprathermal tails of electrons in the solar corona can have a strong influence on the formation of the solar corona and the dynamics of the solar wind.

Solar wind ionic charge states during the Ulysses pole-to-pole pass

We analyze and compare the ionic charge composition data for different types of the solar wind (slow wind from equatorial regions, fast wind from low-latitude coronal hole and fast wind from both south and north polar coronal hole) which the Solar Wind Ion Composition Spectrometer (SWICS) on Ulysses observed during the pole-to-pole pass of its primary mission. The implications on the electron temperature, electron density, and ion outflow velocity from the corresponding solar wind source regions are also discussed. We find that the electron temperature in the source region of the slow solar wind is higher than that in the coronal hole. We also find a possible north-south asymmetry in the electron temperature that may be correlated to the northsouth asymmetry in the solar wind speed found in the SWOOPS/Ulysses data. In particular, we make extensive discussions on the latitudinal variations in the polar coronal hole. On the basis of our data without clear constraints from other coronal observations, the preliminary result is that the electron density may be higher, or the heavy ion outflow velocities may be lower toward lower heliographic latitude.

On the differential ion velocity in the inner solar corona and the observed solar wind ionic charge states

Theoretical calculations on multifluid solar wind have indicated that the velocities of ions of the same element are not the same in the inner coronal region where the freeze-in process of these solar wind heavy ions occurs. This may have nonnegligible effect on the inference of the electron temperature in this region from the observed ionic charge states. In this paper, we investigate the effect of differential ion velocities on the inferred freeze-in temperatures and the coronal electron temperature diagnostics. We then investigate if the ionic charge state data are able to provide constraints on the differential velocities of these ions in their freezing-in region. We use the ionic charge composition data observed by the SWICS instrument on Ulysses during the south polar pass as an example.